It's not an altogether ridiculous notion that if Kieran Tierney was asked to literally run through a brick wall for Celtic then someone would have to step in and restrain him.

Figuratively, he does it every week and has done ever since Ronny Deila flung him on for the final 10 minutes at Dens Park in 2015.

But there are worrying signs that it's beginning to take a toll on a young man still to reach his 22nd birthday.

"You know when he is in pain and he really is in pain - he's a real throwback. Football is his life."

That was Brendan Rodgers back in January, putting into words what Celtic fans have always known about Tierney.

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He's played an incredible amount of minutes since displacing Emilio Izaguirre as first-choice left-back during the 2015/16 season - and it's perhaps beginning to show.

Tierney broke down in December this season, missing the entire festive schedule with a hip injury and there was a suggestion from Rodgers he may have needed an operation.

He managed to avoid the surgeon's knife to return against Motherwell in February, where he played 68 minutes.

He then played the full 90 in each of Celtic's next three fixtures and attempted to go off on international duty with Scotland.

National team doctors sent him back to Glasgow after he sat out the debacle in Kazakhstan but he was straight back into the Celtic team for a blood and thunder derby with Rangers at the weekend.

Changing of the guard: Celtic's Emilio Izaguirre makes way for Kieran Tierney in April 2015

That he pulled up again is probably no surprise, limping off clutching the same left calf that ruled him out of Scotland's Euro 2020 qualifiers - an issue John Kennedy later put down to cramp.

As discussed on Sunday's Record Celtic podcast, which you can listen to below, Tierney's mammoth amount of minutes could be putting him at risk long-term.

It's not pure speculation, either. He was released from Scotland duty in October last year "to avoid burnout", so it's at least been acknowledged internally as a potential issue for the 21-year-old.

And it's little wonder why.

Since breaking into the Celtic team, Tierney has played 14,233 minutes of football for the club, an average of 41 games per season. In January 2018, it was revealed Tierney had played more minutes so far that term than any other top flight outfield footballer in Europe.

Quite the workload for a player still only 20 at the time, regardless of his unrivalled dedication to the Celtic cause.

It's also a rare occasion that Tierney doesn't play 90 minutes, so vital is he to his side.

Here's how those minutes have broken down since his first full season in the Hoops.

2015/16 - 2,860

2016/17 - 3,479

2017/18 - 4,701

2018/19 - 3,193

After the win over Rangers on Sunday, Neil Lennon acknowledged Tierney wasn't fully fit and it was clear to anyone watching that he faded badly after the hour mark.

Days earlier, Lennon had also admitted the youngster was already running on empty, but Celtic's dearth of options at left-back left the manager with little choice but to stick with him.

“He probably needs a rest," Lennon said.

“We’ll try and get him over the line as quickly as possible, then shut him down for a while.

“We’ll be guided by the medical team.”

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Burnout, although the symptoms can be physical, is understood to be more of a psychological issue.

Tierney's willingness to continually put his body on the line show there's no lack of motivation but he's beginning to pick up niggling injuries and Celtic don't want that to become a theme.

Persistent problems can be devastating to a young player's development and particularly in someone such as Tierney, who relies so much on his running power and explosiveness.

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With another Champions League qualifying campaign looming and the early start that brings with it, the final few months of the season could be crucial.

Celtic have the Premiership title in the bag, which could give Tierney some breathing space between now and the end of the campaign.

It probably won't happen this way but conceivably there's only two more games he needs to play in this season - the Scottish Cup semi-final and a potential final thereafter.

It's the ideal opportunity for Celtic to take a player who seems to feeling the physical toll out of the firing line for his own good.

Because if there's one thing that's certain, Tierney's not going to make that call himself.